Medical instruments such as endoscopes and other luminal devices, having long narrow passageways, generally have to be cleaned between uses. Current cleaning methods for cleaning the interiors of long narrow passageways include single-phase liquid flow followed by single-phase gas flow, with the single-phase gas flow mostly used for drying. Use of mixed-phase flow has been disclosed in patent such as U.S. Pat. Nos. 6,027,572 and 6,857,436 and 6,454,871 all to Labib, in a flow regime such that gas-driven droplets of liquid strike contaminants and dislodge them. However, in some situations as in flexible endoscopes, there are pressure limitations which make it impossible or unlikely for gas-driven droplets to form or if formed, their concentration is very low and their velocity is too small to attain sufficient momentum to dislodge contaminants by impact of droplets. A different regime of cleaning has been disclosed in U.S. Pat. No. 4,781,764 to Leenaars, which has used surface tension forces at a moving interface between solid and liquid and gas to remove contaminants from an externally-facing surface of a flat plate. Leenaars' contaminants were inorganic, not biologically adhered, and the motion needed to effect cleaning was relatively slow. U.S. Pat. No. 5,279,799 to Moser discloses limited use of liquid and gas flow separately in cleaning endoscopes. There is industrial literature of two-phase liquid and gas flow but usually involving a wall which remains wet during the two-phase flow. In these respects and also in other respects, there remains room for improvement in both results and ease of performing cleaning.